Male meiosis and gametogenesis in wild house mice (Mus musculus domesticus) from a chromosomal hybrid zone; a comparison between "simple" Robertsonian heterozygotes and homozygotes

Wild male house mice Mus musculus domesticus were collected from the hybrid zone between the John o'Groats race (2n = 32) and the standard race (2n = 40) in northern Scotland. Meiosis in both homozygotes (2n = 32, 36, and 40) and single Robertsonian heterozygotes (2n = 33, 35, and 37) was found to be orderly. At prophase/metaphase I in heterozygotes, a trivalent was formed from the metacentric and two homologous acrocentrics. At pachytene, this trivalent usually had a single side arm at the position of the centromeres, as a result of nonhomologous pairing of the acrocentrics. This side arm persisted into diplotene. Generally only a single chiasma was formed between each acrocentric and the metacentric. Anaphase I nondisjunction frequencies were estimated as 1.5% for the homozygotes and 2.7% for the heterozygotes. The extent of germ cell death between the pachytene and round spermatid stages was 18% greater in heterozygotes than in homozygotes. Our results concur with previous studies which indicate that single Robertsonian heterozygotes in wild house mice have near-normal fertility.

Male common shrews (Sorex araneus) with long meiotic chain configurations can be fertile: implications for chromosomal models of speciation

Two chromosomal races of common shrews (Sorex araneus) were crossed in captivity to generate chain VII-forming complex Robertsonian heterozygotes. Meiosis and gametogenesis were studied in three male hybrids. Regular chain VII configurations were observed at both pachytene and diakinesis/metaphase I, although in many pachytene spreads the chain configuration was incomplete (the basis of this peculiarity is unknown). From metaphase II counts, the frequency of anaphase I nondisjunction in the complex heterozygotes was estimated to be 13%. Germ-cell death in the chain VII-forming complex heterozygotes was 22% greater than it was in controls, but this difference is unlikely to have greatly influenced the capacity of the heterozygotes to sire offspring. Thus, the fecundity of these complex heterozygous common shrews would probably have been only slightly reduced relative to homozygous or simple heterozygous shrews. These results call into question the generality of speciation models based on the presumed sterility of complex heterozygotes.

A hybrid zone comprising staggered chromosomal clines in the house mouse (Mus musculus domesticus)

In the vicinity of John o'Groats (Caithness, Scotland) there is a small karyotypic race of the house mouse (Mus musculus domesticus) characterized by a diploid number of 32 chromosomes, including the metacentrics 4.10, 9.12, 6.13 and 11.14. This race forms a hybrid zone with the standard British race (fully acrocentric chromosomes, 2n = 40). Although hybrid zones normally consist of several (or many) narrow character clines at the same position, this zone is unusual in that the chromosomal clines do not coincide. The cline for arm combination 11.14 is staggered relative to the 6.13 cline and both are separate from the clines for 4.10 and 9.12 (which may or may not coincide). A variety of explanations for the structure of the hybrid zone are discussed. It is possible that this may be a case of 'zonal raciation'.

Spermatogenesis in heterozygotes for Robertsonian chromosomal rearrangements from natural populations of the common shrew, Sorex araneus

Twenty-two adult male common shrews were collected from 5 sites in the vicinity of Oxford (UK) close to the zone of hybridization between two karyotypic races. The shrews were subdivided into 3 karyotypic categories: homozygotes, simple Robertsonian heterozygotes (which form one or more trivalents at prophase I of meiosis) and complex Robertsonian heterozygotes (which form a quadrivalent). The ratio of primary spermatocytes to round spermatids was determined from transverse sections of seminiferous tubules, to provide an indication of germ cell death. In no individual was there severe germ cells loss. Homozygotes had the highest mean spermatocyte: spermatid ratio and complex heterozygotes the lowest, but there was substantial individual variation and the differences were not significant. Complex heterozygotes also had a higher proportion of defective seminiferous tubules and lower testis weights than did other categories and it is reasonable to propose that, as a population, complex heterozygotes had reduced fitness relative to other categories on the basis of spermatogenic performance. However, there is no evidence from studies of spermatogenesis that simple Robertsonian heterozygotes are less fit than homozygotes.

The chromosome complement of Sorex granarius--the ancestral karyotype of the common shrew (Sorex araneus)?

We present the G-band karyotype of Sorex granarius (Miller, 1910). With minor exceptions, each of the acrocentric autosomes in this karyotype is completely homologous to one of the chromosome arms in the karyotype of the common shrew (Sorex araneus L., 1758). If, as is the simplest interpretation, the karyotypic evolution in S. araneus is by repeated Robertsonian fusion, the karyotype in S. granarius represents the ancestral condition. We urge breeding studies to establish whether the designation of S. granarius as a full species is justified.

Factors responsible for a karyotypic polymorphism in the common shrew, Sorex araneus

A Robertsonian karyotypic polymorphism in the common shrew in the Oxford area, first described in the 1950s, was re-examined. The polymorphism involves chromosome arm combinations kq, no and pr (characteristic of the Oxford karyotypic race), ko (characteristic of the Hermitage karyotypic race) and jl (found in both races). The polymorphism for jl was sporadic along a north-south transect through the Oxford area, with the frequency of the twin-acrocentric morph never exceeding 10%. The frequency of the Oxford race-specific metacentrics decreased and the frequency of the Hermitage race-specific metacentric ko increased from north to south along the transect. At a latitudinal grid reference of about 180 km, there was a high frequency of individuals with chromosome arms k, n, o and q in the ancestral acrocentric state. This was coincident with the area of occurrence of ko-kq and ko-no Oxford-Hermitage hybrids. Such hybrids are double Robertsonian heterozygotes with monobrachial homology and are likely to suffer reduced fertility in consequence. It is proposed that this is a source of selection against the monobrachial hybrids and hence results in an increase in frequency of the acrocentric morphs. This scheme goes some way to explain the clines of polymorphism for arm combinations kq, no and ko, but it is suggested that other selective factors are involved. It cannot explain the cline of polymorphism for pr, which is in general terms similar to that for kq and no, but is more shallow and centred further north.

Preferential transmission in wild common shrews (Sorex araneus), heterozygous for Robertsonian rearrangements

Data from foetal and weanling studies suggest that Oxford race metacentrics kq and no, but not pr, are transmitted preferentially from Robertsonian heterozygote common shrews collected from the Oxford area. These results suggest a possible mechanism for the maintenance of the Robertsonian polymorphism in that area.

A trimeric esterase in the common shrew

A variable kidney esterase found in the common shrew has a trimeric structure as indicated by the phenotypes of heterozygotes. From substrate specificity and tissue distribution this enzyme appears to be homologous with esterase-6 of the mouse, also a trimeric enzyme. It is suggested that a trimeric esterase may have been a feature of early eutherian mammals.

Meiotic studies of Robertsonian heterozygotes from natural populations of the common shrew, Sorex araneus L

Adult male common shrews, both Robertsonian heterozygotes and homozygotes, were collected from Oxford and elsewhere in Britain. In both simple Robertsonian heterozygotes and Robertsonian heterozygotes with monobrachial homology, regular chain configurations were observed at meiosis I; only 1-2% were incomplete such that univalents were observed. On the average, there was one chiasma per chromosome arm among those that displayed Robertsonian variation, including both chain configurations and bivalents. According to one hypothesis, a single chiasma per chromosome arm may facilitate proper disjunction of chain trivalents of simple Robertsonian heterozygotes. Based on metaphase II counts, anaphase I nondisjunction frequency can be estimated as 1.0% per heterozygous individual and 0.7% per heterozygous arm combination.

The XYY condition in a wild mammal: an XY/XYY mosaic common shrew (Sorex araneus)

XY/XYY sex-chromosome mosaicism was demonstrated in both bone marrow and germ cells of a wild adult common shrew. Secondary sexual characteristics were those of a normal male, but the testes were small, and the sperm count was only about 3% of normal. Most of the seminiferous tubule cross-sections examined revealed serious spermatogenic impairment and a reduced diameter. A range of sex-chromosome pairing configurations was observed in XYY primary spermatocytes, including configurations involving the X and both Y chromosomes in a linear or radial array. The presence of metaphase II (MII) spreads with an XY sex-chromosome complement indicated that XYY primary spermatocytes could contribute products to MII. Following Burgoyne (1979) and Burgoyne and Biddle (1980), a number of models of spermatocyte loss were tested. The data indicated that there was an association between the sex-chromosome complement of primary spermatocytes and their contribution to MII. The best fit to the observed MII frequency data was provided by a model which assumed that all XYY primary spermatocytes with a univalent Y chromosome and a high proportion of XYY primary spermatocytes with an unpaired X chromosome failed to contribute products to MII.

Isoenzyme variation in the common shrew (Sorex araneus) in Britain, in relation to karyotype

Samples of common shrews with an Aberdeen race, Oxford race and Hermitage race karyotype and from the Oxford-Hermitage hybrid zone were screened for genotype at six polymorphic enzyme loci. The common allele at all loci was the same in all samples suggesting that the degree of genic divergence between the karyotypic races is not great. However, shrews of the Aberdeen race appear to be somewhat distinct. There were differences in allele frequencies, range of alleles and heterozygosities at the Mpi-1, Pgm-2 and Pgm-3 loci between the Aberdeen race samples and samples of other karyotypic categories. The allele frequency variation across the Oxford-Hermitage hybrid zone is not substantially linked with karyotype frequency change, but there is notable allele frequency variation between close sites of similar habitat, particularly at the Pgm-2 and Pgm-3 loci. This suggests that gene flow between close sites may be reduced sufficiently in some instances to allow allele frequency change by genetic drift. This may have implications for the mode of origin of karyotypic races of common shrew.

Breeding the common shrew (Sorex araneus) in captivity

A method for the systematic breeding of the common shrew (Sorex araneus) is described. By use of this method 20 litters of common shrew, conceived in the wild and in captivity, were reared to weaning. Successful fertilization was recorded for 74% of pairings and 68% of young known to have been born were reared to weaning. Breeding records indicate that common shrews will mate within a few days of pairing and that the duration of pregnancy and lactation is about 43 days.

A wild common shrew (Sorex araneus) with an XXY sex chromosome constitution

An adult male common shrew with an XXY sex chromosome constitution was found in a natural population. The external appearance of the animal and size of the accessory glands were normal but the testes were as small as those of immature shrews. Histological preparations of the testis revealed seminiferous tubules containing only one type of cell, probably Sertoli cells, and interstitial cell hyperplasia.

Nondisjunction frequencies in Robertsonian heterozygotes from natural populations of the common shrew, Sorex araneus L

Pregnant female common shrews were collected from an area of Robertsonian polymorphism, involving five different arm combinations, around Oxford (England). The females and their fetuses were karyotyped, and the karyotypes of the sires were deduced. Ten pregnancies where at least one parent was known to be either a single or double Robertsonian heterozygote were available for analysis. From these pregnancies, upper and lower estimates of anaphase I nondisjunction arising from Robertsonian heterozygosity were calculated to be 1.0-2.5% per heterozygous arm combination and 1.5-3.7% per heterozygous parent. One trisomic fetus with retarded development was identified. This trisomy can be attributed to anaphase I nondisjunction of a trivalent in a female Robertsonian heterozygote.